Pretreatment for esterification of cellulose in attenuated form



March 14, 1939. I Q J, M 2,150,690. PRETREATMENT FOR ESTERlFICATION OFCELLULOSE IN ATTENUATED FORM Filed 001;. 14, 1937 I HE. II.

' Carl J. Malm JNVENTOR fiMM W ATTO' EYS Patented Mar. 14, 1939 UNITEDSTATES PATNT OFIQ PRETR-EATMIENT FOR ESTERIFICATION 0F CELLULOSE INATTENUATED FORM Application October 14, 1937, Serial No. 168,966

9 Claims.

The present invention relates to the treatment of cellulose inattenuated form, such as a sheet, with a suitable pretreatment bath,such as one consisting of one or more lower fatty acids of which atleast 40% is acetic acid, enclosing the treated cellulose in suitablecontainers and allowing it to stand until activated to the desiredextent. The cellulose may then be acylated.

This application is a continuation-in-part of my application Serial No.81,323 filed May 22, 1936.

Ordinarily in the manufacture of cellulose acetate or other celluloseesters, it is customary to pretreat the cellulose with a lower fattyacid either with or without a small amount of catalyst. The pretreatmentof the cellulose is usually carried out in the same vessel in which thesubsequent esterification is carried out. This method has thedisadvantage of requiring the use of an esterification vessel for arelatively long period of time for each batch of cellulose acetate sothat the production in the equipment is restricted thereby. If, on theother hand, a separate vessel is employed for the pretreatment of thecellulose, difficulties arise in transferring the pretreated cellulosequantitatively from the pretreatment bath to the reaction vessel.

One object of my invention is to provide a pretreatment for cellulosehaving a shorter mixer cycle and thereby greatly increasing the capacityof a given amount of esterification equipment. Another object of myinvention is: to provide a process in which the cellulose, which is tobe used for making cellulose esters containing higher acyl will get abetter pretreatment. Another object of my invention is to provide apretreatment process which is moreefiicient and easily controlled thanthat carried out with an excess of liquid. A still further object of myinvention is to provide a method of pretreatment in which less handlingof the cellulose is necessary than was formerly the case in thepreparation of cellulose esters.

I have found that cellulose may be uniformly pretreated by contacting asheet of the cellulose with a pretreatment bath consisting of one ormore lower fatty acids containing at least 40% of acetic acid and then,with the sheet in only a moist condition, storing it in a closedcontainer at the temperature and for the length of time desired. It ispreferable to compact the treated sheet, such as by rolling it uptightly or cutting it and piling the cut sheets, before or at the timeit is put into the container thus further economizing upon equipment.Thus it becomes unnecessary to use the esterification vessel inpretreating the cellulose. The cellulose may thereafter be introduceddirectly into the reaction mixture without any serious physicaldisintegration being necessary. Also the cellulose may be given a mostsatisfactory pretreatment without a large excess of acetic acid beingpresent therein, which is quite important where the subsequentesterification involves the use of reagents in a high proportion to theacetic acid.

My method of pretreating cellulose may consist in providing thecellulose in the form of a continuous sheet or batting and treating thesheet with a lower fatty acid containing a large proportion of aceticacid, either by passing it through the lower fatty acid bath or over aroll moistened with the acid and, if necessary, squeezing the sheet sothat about A; to 2 parts by weight of the liquid remains therein forevery part of cellulose, after which the sheet is, for instance, woundtightly into a roll and the roll is stored either alone or with otherrolls in a closed container at the desired temperature and for thedesired length of time. This treatment activates the sheet so that it isreadily reacted upon by the esterification mixture and weakens the sheetsufficiently so that it is: easily disintegrated upon being introduceddirectly into the esterification vessel containing a stirring device.Thus long soaking of the sheet in an excess of acid is avoided and theamount of pretreatment equipment is reduced to a minimum. The sheet isjust as effectively pretreated by my novel method as by soaking inexcess acid for the reason that the acid vapors in the enclosed vesselthoroughly permeate every fiber and do the Work of a great excess ofliquid with much greater economy.

Instead of rolling up the sheet, it may be folded, or cut into piecesand stacked, or some other procedure may be used to bring it togetherinto a compact mass.

It is desirable in order to obtain uniform pretreatment that the acidwith which the sheet is treated be kept at the temperature at which thepretreatment is to be carried out. This pretreat ment temperature isdetermined by the amount of activation or the decrease of thecuprammonium viscosity of the cellulose in the pretreatment which it isdesired to obtain. For instance, in the manufacture of cellulose acetatewhere a small amount of catalyst is to be employed, a cellulose of lowviscosity is desirable as the starting material for the esterificationin order to prevent gelling of the dope formed. Unless the cellulose forpreparing cellulose acetate is a low viscosity material, thepretreatment should be carried out for a longer time or at a highertemperature, such as about 180-220 F., unless large amounts of catalystare used in the esterification. If, however, the pretreatment is to befollowed by another pretreatment employing a catalyst before theacetylation (as I may sometimes do), such a high temperature would beunnecessary as the second pretreatment with a catalyst will serve tolower the viscosity. Where the cellulose is to be used in anesterification in which gelling of the reaction mixture after theesterification is not liable to occur, such as in the esterification ofcellulose in a bath containing propionyl or butyryl or a chlorinatedhydrocarbon such as ethylene chloride or methylene chloride as thesolvent, or when a cellulose having a low viscosity is pretreated, thefunction of the pretreatment should be primarily to swell the fibers andmake them more readily susceptible to esterification rather than tolower the cuprammonium viscosity. In. this case where the lowering ofthe cuprammonium viscosity is not necessary, shorter time ofpretreatment or lower pretreatment temperatures, such as 100 F. orordinary room temperature (70 F.) or some temperature therebetween maybe employed. If it is desired to retain the viscosity of a highviscosity cellulose the procedurefor merely swelling the fibers onlyshould be employed. In the latter two instances one would be guidedmerely by the extent of activation of the cellulose necessary ordesirable in such instances; experience with subsequent esterificationreadily indicates the amount of activation 'required for the specificcellulose being employed. My pretreatment method may be employed forpretreating any type of cellulose which can be formed into sheets havingsuflicient mechanical strength to be led over rollers and wound into asheet as will be described hereinafter. At the present time, cotton andrefined wood pulp, such as refined sulfite pulp, are the most commonlyemployed starting materials for the preparation of cellulose esters.Wood pulp and cotton linters are frequently prepared in sheet form and,therefore, are particularly adaptable to my process.

The liquid employed for pretreating in accordance with my inventionshould contain a considerable proportion of acetic acid. If eitherpropionic or butyric acid are present it, is necessary that the fattyacids of more than 2 carbon atoms constitute not more than of theliquid. It is desirable that the pretreatmentliquid be concentrated acidand, for economical reasons, that it contain not more than 10% of asubstance that will react with acetic anhydride, such for instance aswater or alcohols. The remainder of the liquid should constitutelowerfatty acid except in some cases where it is desirable to add asmall amount of acylation catalyst such as concentrated sulfuric acid tothe pretreatment liquid.

Thus, it is preferred that the pretreatment liquid be nearly anhydrousas the moisture in the cellulose will tend to dilute the acid to someextent; it is preferable that it should be of at least 90% concentrationunless the cellulose should have such a small moisture content that nodilution from that moisture would occur. In that case the concentrationcould be a few per cent below 90% and an economical pretreatment wouldoccur. In other words, my pretreatment method is effective with anyconcentration or mixture of lower fatty acids and still obtain theresult of low equipment cost; however, the limitations given above havebeen found to be the most -or temperature of the pretreatment has to beincreased in order to obtain the desired activation. If desired anacylation catalyst may be present in small amount in the pretreatmentbath especially if appreciable reduction of the viscosity of thecellulose is desired. The catalyst may be sulfuric acid, perchloric acidor the like and may be present in an amount up to 5% of the pretreatmentliquid. Apparatus which may be employed for pretreating cellulose, inaccordance with my invention, is illustrated by the attached drawing.The method of impregnating the cellulose may be governed by the amountof pretreatment acid which it is desired should be in direct contactwith the cellulose during the time of treatment. The cellulose sheet maybe treated either by passing the sheet through the pretreatment acid(soaking treatment) followed by squeezing out the excess and/or bypassing it over a surface which has been moistened with pretreating acidfrom which it 'will absorb the liquid (contact treatment). The soakingtreatment is primarily adapted to a treatment in which it is desiredthat the cellulose contain 1 to 2 parts by weight of the pretreatingliquid for every part of cellulose. If, however, the use of a smallercontent of the liquid in the cellulose is desired, such as about /2 partor even less of acid to every part of cellulose, the method, in whichthe cellulose sheet contacts a surface wet with the pretreating acid issuitable. Fig. 1 illustrates an apparatus in which the soaking of thecellulose sheet in the pretreating liquid is accomplished. The cellulosesheet I which is to be pretreated comes rolled-up on the feed roll 2from which it is led into the apparatus. This is done by feeding the endof the sheet into a guide funnel 3 where the drive rolls 4 start thesheet on its course through the apparatus. In order to direct the sheetthrough the apparatus, guide plates 5 are provided. There are severalsets of these plates provided to direct the sheet. It is preferred thatthe outer guide plates extend beyond the edges of the sheet and that thespace through which the sheet moves be closed by a vertical strip tolimit the motion of the sheet in a sidewise direction. After the sheetpasses through the drive rolls and into the guide plates, it is led intoa vat 6 containing the pretreating liquid such as glacial acetic acid. Asufficient level of the pretreating acid to assure good soaking ismaintained by conducting the acid as needed into the vat through thefeed pipe I. The vat is also provided with an overflow 8 to prevent thelevel of the acetic acid becoming too high. It is preferred that thesheet contact the pretreating acid long enough to thoroughly saturatethe sheet. The temperature of the bath should The soaked cellulose sheetis then led to the squeeze rolls 9 by which the excess acid is removed.By the arrangement shown, the excess acid squeezed out is returned tothe vat contain-9 ing the pretreating "acid. The squeeze rolls shouldpreferably admit of adjustment so that the pressure which is exerted bythem upon the sheet can be varied in accordance with the amount of acidwhich it is desired should remain in the sheet. From the squeeze rollsthe sheet is further directed through the guide plates and to the slotin the wind-up roll ID by means of the swinging guide plates 14 whichare hinged at points l5. This wind-up roll is driven by a drive roll llof the floating-friction type. The passage of the sheet through the slotof the wind-up roll may be noted by a photo-electric cell unit It atthis point. By this means the rotation of the drive roll H may bestarted at the proper moment and also the guide plates I4 may be swungclear of the roll of cellulose sheet upon the wind-up roll in asitincreases in size. Instead of having a photo-electric unit and automaticcontrol, starting of the rotation of the drive roll H and the swingingup of the guide plates M may be manually operated. When the entire sheethas passed through the apparatus and has been wound upon the wind-uproll, it may be removed from the apparatus through the hinged air-sealplate l3. As a slightly decreased pressure is preferably maintainedwithin the apparatus, this air-seal plate is desirable to avoid drawinga large quantity of air into the apparatus. The roll after removal fromthe apparatus is placed in a closed container adapted to contain eitherone roll or a number of rolls. Tofacilitate handling, it is desirablethat the container be mounted on wheels so that it may be easily movedabout. After the roll has been enclosed for the required time and at thedesired temperature, the sheet may be then led into an esterificationmixer to be esterified forming an organic acid ester of cellulose.

'Fig. 2 illustrates a modified apparatus in which the cellulose sheetmay be moistened with the pretreating acid instead of being dipped orsoaked. This would make unnecessary the vat 6 and the squeeze rolls 9which are necessary in the dipping process. This modified treatingapparatus consists of a vat of pretreating acid provided with a cover 2|with an opening sufficient for the top part of the treating wheel 22 toprotrude. It is preferred that there also be an idling roll 23 directlyabove the treating wheel 22 to ride upon the surface of the cellulosesheet and thus assure a good contact between the sheet and the acidpresent on the roll 22. The amount of. acid which will be taken-up bythe cellulose sheet, using this apparatus, will depend uponthe surfacespeed of the roll, the surface of the roll and the level of thepretreating acid in the vat. Thus, increasing the speed or increasingthe level of the liquid would result in increased soaking-up of the acidby the cellulose sheet. In order to still further increase the amount ofpretreating acid presented to the cellulose sheet by roll 22, anembossed surface may be employed.

In the use of the apparatus of Fig. 1, it is desirable that the driverolls 4, the squeeze rolls 9 and the wind-up rolls Ill be synchronizedto run at a constant surface speed. The surface speed of the wind-uproll It) depends on the drive roll II and does not .Vary with the sizeof the roll which has collected on wind-up roll l0.

Although the operation of this apparatus has been explained withreference to the processing of individual sheets, the operation may bemade continuous by providing a cellulose sheet of continuous length. Inthat case, of course, after the wind-up roll It) contains a roll oftreated cellulose of desired size, it will be necessary to cut the sheetat this point and insert a new wind-up roll. The following examplesillustrate processes in which cellulose sheets are pretreated inaccordance with the present invention:

Example I 25 lbs. of refined wood pulp, having a high alpha cellulosecontent which is in the form of a roll of a narrow sheet of considerablelength was passed through a bath containing glacial acetic acid (at roomtemperature) and through squeeze rolls which were adjusted so that thesheet after rewinding into a roll weighed approximately lbs. The rollwas then kept in an air-tight, stainless-steel container for about 12hours at room temperature. This sheet was then led off from this rollinto a mixer containing 10 lbs of acetic acid, 140 lbs. of propionicacid, 75 lbs. of 85% acetic anhydride and 110 cc. of sulfuric acid, thereaction mixture having previously been cooled to F. The reactiontemperature was permitted to rise to a maximum temperature of about100-110 F. over a period of about 4 hours whereupon a clear viscoussolution of cellulose acetate propionate was obtained. The resultingester may be hydrolyzed or directly precipitated out in any desiredmanner, such as by pouring into aqueous aceticacid as described andclaimed in my Patent No. 2,030,883.

Example II 25 lbs. of cellulose, having a 2 cuprammonium viscosity of250 centipoises, which was in sheet form and upon a roll, was led intoan acetic acid bath kept at a temperature of about 180 F. The excessacetic acid was squeezed out leaving 25 lbs. of acid in the sheet and itwas Wound tightly onto a roll and kept at about 180 F. for approximately6 hours. into an esterification vessel containing 150 lbs. of glacialacetic acid, '75 lbs. of 85% acetic anhydride and 80 cc. of sulfuricacid which mixture had been cooled to 55 F. The temperature Was allowedto rise to 90-95 F. over a period of approximately 3 hours. Aftertreating for another hour a clear and Viscous dope consisting ofcellulose acetate in solution in the spent esterification mixture wasobtained.

The length and width of the cellulose sheets employed in. my inventionmay be those found most convenient. The width, for instance, would begoverned by the width of the dipping apparatus and by the size of theopening of the mixer in which the acetylation is carried out. The lengthof the sheet would depend upon the roll size which could be most readilyhandled. For the ordinary mixer a sheet having 20" width has been foundto be quite suitable.

It is preferred to employ a cellulose sheet, dried down to a lowmoisture content, such as about 2%. The roll of cellulose sheet may bestandardized as to size at that desirable for a single charge ofcellulose for the esterification mixer. If the mixers are small, thecharge of cellulose used may be from 25 to 100 lbs. If theesterification mixers are of good size, the roll of cellulose sheetmight be as great as 500 lbs. However, instead of having the entirecharge for a mixer in one sheet, it might be desirable to use The sheetwas then led smaller rolls and employ more than one roll of treatedcellulose sheet for each charge.

After the pretreatment has occurred, it is probably desirable to knowthe amount of pretreating liquid which is present therein, to aid inmaking-up an esterification bath of the desired composition. This may bedetermined by weighing the treated roll, the amount of the pretreatingliquid present therein, being the difference between that weight and theweight of the cellulose sheet before its treatment.

Thus, I have provided a pretreatment method requiring a minimum ofequipment and space, a minimum of pretreating liquid and a minimum ofhandling of the cellulose. Regardless of the kind of pretreating liquidemployed and regardless of the amount of liquid initially deposited inthe cellulose sheet, the liquid difluses and vaporizes and thoroughlypermeates every fiber of cellulose during the time that it is containedin the enclosed vessel in which it is put. I have further found that theeffectiveness of the pretreatment is not determined by employing largeramounts of pretreating liquid; in other words the dipping step whichmore thoroughly saturates the cellulose fibers does not give any moreefiective activation of the cellulose than does the contact step wherethe cellulose is allowed to take up only a small amount of pr-etreatingliquid, nor is the time or temperature of pretreatment changed by sodoing. In fact where a catalyst is employed in the pretreating liquid,the contact method is preferable because it allows a minimum ofselective absorption of the catalyst from the organic acid anddistributes the catalyst more evenly in the cellulose.

Compacting the cellulose following impregnation with pretreating liquidis not necessary for effectiveness of the process but is desirable tocut down the number and size of containers necessary to handle a givenquantity of cellulose. In other words the cellulose containingpretreating liquid may be placed loosely in the vessels, stacked invessels, rolled and placed in vessels or tamped in, depending upon theequipment available. The only essential step is that the cellulosecontaining pretreating liquid be enclosed in a chamber or vessel for atime and at a temperature sufiicient to make it readily esterifiable.

I claim:

1. The method of pretreating cellulose to prepare it for esterificationwhich comprises impregnating cellulose in attenuated form with apretreatment liquid essentially consisting of lower fatty acid of whichat least 40% is acetic acid, enclosing the impregnated cellulose in acontainer and allowing it to stand for the desired time and at thedesired temperature until the cellulose is sufficiently pretreated foresterification.

2. The method of pretreating cellulose to prepare it for esterificationwhich comprises impregnating cellulose in attenuated form. with a bathessentially consisting of concentrated lower fatty acid of which atleast 40% is acetic acid, enclosing the impregnated cellulose in acontainer and allowing it to stand for the desired time and at thedesired temperature until the cellulose is sufficiently pretreated foresterification.

3. The method of pretreating cellulose to prepare it for esterification.which comprises impregnating cellulose in attenuated form with a bathessentially consisting of concentrated acetic acid, enclosing theimpregnated cellulose in a container and allowing it to stand for thedesired time and at the desired temperature until. the :ellulose issufliciently pretreated for esterifica- 4. The method of pretreatingcellulose to prepare it for esterification which comprises impregnatingcellulose in attenuated form with a pretreatment liquid essentiallyconsisting of lower fatty acid of which at least 40% is acetic acid,compacting and enclosing the impregnated cellulose in a container andallowing it to stand for the desired time and at the desired temperatureuntil the cellulose is sufiiciently pretreated for esterification.

5. The method of pretreating cellulose to prepare it for esterificationwhich comprises impregnating cellulose incattenuated form with a bathessentially consisting of 4-2 parts of concentrated lower fatty acid ofwhich at least 40% is acetic acid, enclosing the impregnated cellulosein a container and allowing it to stand for the desired time and at thedesired temperature until the celluloseis sufficiently pretreated foresterification.

6. The method of pretreating cellulose to prepare it for esterification.which comprises impregnating cellulose in attenuated form with a bathessentially consisting of %,-2 parts of concentrated lower fatty acid ofwhich at least 40% is acetic acid and a small amount of acylationcatalyst, enclosing the impregnated cellulose in a container andallowing it to stand for the desired time and at the desired temperatureuntil the cellulose is sufiiciflently pretreated for esterification.

'7. The method of pretreating cellulose to prepare it for esterificationwhich comprises impregnating cellulose in attenuated form with a bathessentially consisting of lower fatty acid of which at least 40% isacetic, forming the cellulose into a mass, enclosing it in a containerand allowing it to stand at approximately 100 F. until the cellulose issufficiently pretreated for esterification.

8. The method of pretreating cellulose which comprises contacting acellulose sheet with concentrated acetic acid so as to leave 2 parts ofacetic acid by weight for every part of cellulose,

winding the sheet into the form of a roll and allowing the roll to standat 100 F. for a desired time.

9. The method of pretreating cellulose which comprises contacting acellulose sheet with a pretreatment bath comprising concentratedaceticacid and a small amount of acylation catalyst so as to leave 4-2 partsof the bath for every part of cellulose, winding the sheet into the formof a roll and allowing the roll to stand at 100 F. for a desired time.

' CARL J. MALM.

